Parametric devices are flexible and convenient sources of widely tunable coherent radiation. In these, a coherent beam of electromagnetic radiation is applied to a nonlinear optical crystal so as to stimulate a nonlinear optical process resulting in the division of the power/energy in this coherent pump wave into two generated waves, typically referred to as the signal and idler waves. The signal is usually defined as that wave providing the useful output, and as such throughout this document is identified as the wave having the longer wavelength of the two generated waves.
There is considerable interest in extending the spectral coverage of parametric devices. This is because they are often used as sources of coherent radiation in spectral ranges either not covered by any other source types, or where a single parametric-wave source is capable of replacing a number of sources that would otherwise be needed in order to provide the spectral coverage required. However, a limitation of known parametric devices is the detrimental effect of absorption of one or more of the three waves involved in the nonlinear interaction within the nonlinear medium itself. As a result the spectral coverage attainable through a particular parametric generation scheme is often limited only by the presence of absorption and not by the nonlinear or phase-matching characteristics of the nonlinear medium being employed. Mitigation of the restriction imposed by absorption results in improved spectral coverage being attained by parametric-wave devices.
A class of device in which the spectral coverage of parametric generators has been extended is the terahertz (THz) optical parametric generator (OPG), where the useful output (the signal wave) is a beam of wavelength consistent with THz frequencies. Often in such devices absorption of the signal wave is prevalent as the nonlinear medium can be highly absorbing at THz frequencies. A particular example of this type of device includes a non-collinear phase-matching scheme in which the signal wave is made to rapidly walk-out of the nonlinear medium in a direction that is substantially lateral to the propagation direction of the pump wave, hence minimizing the deleterious effects of absorption on the signal wave. Examples of this technique are described in the articles “Efficient, tunable optical emission from LiNbO3 without a resonator”, by Yarborough et al, Applied Physics Letters 15(3), pages 102-104 (1969), “Terahertz wave parametric source”, by Kawase et al, Journal of Physics D: Applied Physics 35(3), pages R1-14 (2002) and “Compact source of continuously and widely-tunable terahertz radiation”, by Dunn et al, Opt. Exp. 14 (4), p. 1582, (2006).
FIG. 1 is an illustration of this known non-collinear phase-matching process. More specifically, FIG. 1(a) illustrates the geometry of the interacting pump 1, idler 2 and signal 3 waves in the nonlinear medium 4. FIG. 1(b) illustrates the phase-matching process through a so-called k-vector diagram, where kp, ki and ks are the wavevectors of the pump, idler and signal waves respectively, angle θ is the angle subtended by the pump and idler waves and angle φ the angle subtended by the pump and signal waves.
As can be seen from FIG. 1(a), in the known non-collinear phase matching process the pump wave 1, the idler wave 2 and the signal wave 3 are non-collinear within the nonlinear medium 4. Thus, to maintain the desired nonlinear interaction between the pump and idler waves throughout the length of the nonlinear medium 4, they must be of sufficient radial (transverse) extent to maintain a spatial overlap between them throughout the length of the medium 4. Furthermore, to maintain the nonlinear interaction between the pump and the signal waves over a length to achieve the necessary parametric gain for the device to work the radial extent of the interacting beams has to be sufficiently large. However, this is contrary to the radial extent of the interacting beams being small for the purpose of increasing the intensities of the beams, so as to reduce the pump power or energy necessary for attaining a level of parametric gain required for the operation of the device.
Another example of the THz OPG class of device is the hybrid collinear/non-collinear phase matched OPG based on slant-stripe periodically poled nonlinear materials, where again the signal wave propagates substantially laterally (and can be orthogonal) to the pump beam direction, see EP1771765, the contents of which are incorporated herein by reference. FIG. 2 shows an example of this known hybrid phase matching process. More specifically, FIG. 2(a) illustrates the geometry of the interacting pump 1, idler 2 and signal 3 waves in the nonlinear medium 4. FIG. 2(b) illustrates the phase-matching process through a k-vector diagram, where kp, ki and ks are the wave vectors of the pump, idler and signal waves respectively, angle α is the angle subtended by the pump and periodic poling grating normal and angle φ the angle subtended by the pump and signal waves.
As can be seen from FIG. 2(a), in the hybrid collinear/non-collinear phase matching process the pump wave 1 and idler wave 2 are themselves collinear within the nonlinear medium 4. However, the signal wave 3 rapidly walks away from both the pump wave 1 and idler wave 2 and rapidly exits the nonlinear medium 4 to avoid excessive absorption loss. Again, there exist the contrary requirements of small radial extent to the interacting beams to minimize the effects of absorption on the signal wave and achieve the pump intensity necessary to achieve a parametric gain sufficient to make the device operate and large radial extent to maximize the interaction length.
In the case generally of parametric generators of non-collinear phase-matched type, and collinear phase-matched type where there can be present a form of walk-off between the interacting waves commonly termed double refraction, these contradictory requirements on the spatial extent of the interacting beams can be addressed by the use of elliptical beam shaping using cylindrical beam shaping optics. In such devices, the cylindrical optics shape the transverse spatial extent of the interacting beams so that the extent of the beams in the non-walk-off plane is made small while the extent of the beams in the walk-off plane is made sufficiently large to maintain spatial overlap between the beams over the length of the nonlinear crystal, but at the same time the overall area of the beams is made desirably small. Thus, the pump intensity can be desirably high.
Optimizing the spatial extent of the interacting beams to maximize the usefully output coupled power/energy in the preferred signal or idler beam is, in the prior art, applicable to systems where absorption is not significant and dependent only upon the desired operating pump beam intensity and degree of non-collinearity. However, this is not directly applicable where one of the beams, normally that associated with the large walk-off direction, is subject to absorption in the nonlinear medium.